195-DTD : Recommendation for test methods for autogenous deformation and thermal dilation of early age concrete
Technical Committee 195-DTD
To improve the performance and competitiveness of concrete as a structural material, great effort has been invested in the period after 1980 to develop a new generation of High Strength and High Performance Concrete (HSC/HPC). The dramatic rise in strength is echoed by the reduction of porosity in the paste. The densification of the material has reduced the permeability by a number of decades. The low-porosity characteristics answered the demand for high performance materials for durability (HPC). However, when the industry adopted HSC/HPC characterised with rather low water/binder ratios, they realised their proneness to early age cracking. This lack of robustness during the hydration phase, which raised questions about the durability, has later represented a major "stumbling block" when introducing HSC/HPC in the market. Research on early age cracking is presently a "hot topic" world wide, and in the 1990's a number of analytical tools based on the "Finite Element Method" have been developed to analyse the cracking risk due to early age volume changes and the internal and external restraint. An example is the Brite EuRam project IPACS (1997 - 2001). The work revealed that the most important reason for the proneness to early age cracking is that the driving forces to self-induced stress generation (and thereby to cracking) during concrete hydration is higher in HSC/HPC. The main driving forces are thermal dilation (TD) and autogenous deformation (AD). TD is higher because the hydration generated temperature rise becomes higher due more binder, but also because HSC/HPC often is used in more massive structures which generates more heat. AD, which is a consequence of the self-desiccation caused by the chemical shrinkage connected to hydration, is higher because low w/b (which is characteristic for HSC/HPC) gives smaller capillary pores, and thus, increased self-desiccation. From an operational point of view it is relatively easy to measure the sum AD+TD for a given HPC and temperature history, and thus to carry out calculations according to the mentioned FEM-programs for a particular case. However, separate models for AD and TD are required to eliminate or at least minimise testing programs in practical applications, and to make the FEM-programs generally valid. In these attempts the researchers have realised that young cement-based materials do not obey traditional material models for TD and AD (concerning the influence of time and temperature). This lack of generally valid models have made it difficult to characterise these material compositions and is also a hindrance in developing the above-mentioned FEM programs to operate in a reliable way. Reliable test methods are necessary in order to be able to develop the material models but also to establish a better fundamental understanding of the two volume changes which is basis for further development of HSC/HPC. In this respect, it is quite frustrating that AD as presented in different publications, e.g. the STAR from RILEM TC 181-EAS "Early Age Shrinkage Induced Stresses and Cracking in Cementitious Systems", and in a Round Robin test in IPACS, varies enormously both in sign (i.e. from contraction to expansion) and magnitude. The influence of temperature varies even more. Similarly, the apparent TD coefficient demonstrated strong dependency on the state of self-desiccation and, in particular, on the detailed method used for its determination. It shows that it is of major importance to discuss measurement techniques before going too far on interpretations of results, and this is the scope of the work in the present committee. The overall objective is to prepare recommendation for measuring methods and procedures for early age autogenous deformation and thermal dilation
Terms of reference
The committee comprises a selected membership of those relatively few organisations and research centres world-wide that have been active in searching for a basic understanding and modelling of the early age deformations of cement based materials. It includes universities, research institutes, material suppliers, contractors, building owners, public agencies and representatives from international associations and standardisation bodies. The members have been recruited from participants in international symposiums and workshops on the topic and from RILEM TC-181 "Early Age Shrinkage Induced Stresses and Cracking in Cementitious Systems". The recommendation will be based on the following main parts: - The STAR prepared in RILEM TC-181 forms the basis for the work - Collection of data from relevant work, in particular on effect of parameters connected to testing equipment and procedures - The committee represents also the great majority of ongoing RTD-projects in this field. The committee will co-ordinate, as far as possible, the relevant parts of these projects - A Round Robin test will be organised and the results discussed Estimated duration of the work is 3 years.
The work will draw on the work in RILEM TC-181 "Early Age Shrinkage Induced Stresses and Cracking in Cementitious Systems". The work will provide input to relevant standardisation-work within CEN, ISO and NIST as well as the pre-standardisation work within fib. The TC fits into the RILEM's technical programme, in particular the items "Performance, maintenance and repair of building materials and structures during the service life" and " Updating of test methods".
The main deliverables are - harmonised test methods, recommendations and - a symposia The work will be based on a considerable amount of data which also will form basis for a data base on autogenous deformation and thermal dilation of young concrete.
Group of users
- academics working with fundamental materials properties and numerical simulation of materials properties - testing laboratories - industrialists working with e.g. materials development - structure designers
Specific use of the results
The results will contribute to a more precise design of concrete structures with respect to the risk of early age cracking, and thus, to extended life time and less repair of concrete structures. The economic impact is, thus, significant. Also, the work will improve the fundamental understanding of the early age volume changes of cementitious systems which is a requisite to design more robust concretes and to the further development of e.g. HSC/HPC with a much better control of the early age cracking risk.